Treatment of ferrous metals to improve resistance to rusting



Patented Dec. 2, 1.947

iJNlTED STATES FFICE TREATMENT OF FERROUS METALS TO IM- PROVE RESISTANCE TO RUSTING Gerson Kurt Bergstein, St. Johns Wood, London, England, assignor, by mesne assignments, to Despo Manufacturing Co. Limited, London, England, a British company No Drawing. Application February 17, 1944, Se-

rial No. 522,802. In Great Britain April 29,

3 Claims. 1

This invention relates to improvements in the treatment of ferrous metals.

One object of the invention is to provide processes for so treating the surfaces of ferrous articles capable of rusting that the development of rust is inhibited.

Processes are known for treating ferrous articles such as iron or steel in order that serious rusting shall not take place during ordinary storage. The object of such processes is not to make the articles resistant to rusting under all conditions of storage and use but simply to provide a surface which is more resistant to rusting than the clean metal and which can, if necessary, be provided with an oil or lacquer coating so that the surface will not become seriously rusted during ordinary storage. In one well known process of this kind articles have to remain in a bath containing phosphoric acid for more than 1 hour and the finish comprises a dull matt surface which is not always desirable. Moreover, in such processes a com- 'plicated and expensive plant is required' A further object of the present invention is to provide a simple process which can be operated at low temperatures and without the use of expensive plant or equipment.

It is known that when an iron surface is treated so as to produce a layer of ferroso-ferric-oxide thereon either by strong oxidation of the surface or by first coating the surface with ferrous oxide and then oxidising this to ferroso-ferricoxide, the resistance to rusting is considerably enhanced. The direct production of ferroso-ferrip-oxide requires the use of a high temperature. Only at about 220 C. does the iron surface begin to oxidise in the air at an appreciable speed but the resulting layer of ferroso-ferric-oxide is formed as a very thin, strongly adhering layer whose colour depends upon the thickness and varies between pale yellow and dark bluish black. It is known, also, that certain combinations of inorgan c salts will bring about this oxidation but temperatures of about 300 C. are necessary requiring skilled operators and these high temperatures are undesirable for many articles.

A further object of the present invention is to ,provide a simplified process of forming a coating of ferroso-ferric-oxide on the surfaces of ferrous articles. f

{ With these and other objects in view the process of the present invention comprises subjecting ferrous articles to the action of an alkaline bath containing an oxidising agent and an alkali metal silicate at a temperature above 100 C. but not above 200 C. Preferably the bath comprises an aqueous alkaline solution of the oxid zing agent containing an alkali metal silicate and of such concentration that it can be heated to a temperature between and 200 C. preferably between and C. without boiling. Oxidizing agents comprising alkali metal compounds are particularly suitable and accordingly there may be used an alkali metal nitrate. bath is a proportion of alkali metal nitrite. Other suitable oxidizing agents are alkali metal (such as sodium) chlorate, permanganate or peroxide. An example of a suitable organic oxidizing agent is nitrophenol. Other oxidizing agents capable of functioning in alkaline solution may be used such as bichromates.

For example, a bath suitable for the present invention may consist of caustic soda and sodium silicate with a smaller proportion of sodium intrate and sodium nitrite and a quantity of water rather less than the weight of these inorganic solids, The resulting mixture will liquefy at a temperature of 125 to 150 C. and the articles to be treated can be immersed in the bath, for example, in a, wire basket for a time of from 5 to 20 minutes and after removal from the bath can be cooled and washed and then dried in a current of warm air.

The resulting coating of ferroso-ferric-oxide is deep black and may be polished or matt according to the conditions of the surface which was treated. The final blackened surface may be treated with any suitable lacquer or can if de sired simply be wiped withoil.

It has been found that the results are improved by adding a small quantity of an lron salt, for example, ferric nitrate, to the bath.

It has further been found that an improvement is effected by adding a heavy metal oxide or cyanide to the bath; the addition of urea to the bath further improves the effect. Alternatively tannic acid may be added, also though less preferably, potassium cyanide and litharge. These additions have the advantage of giving a more uniform coating and also causing the bath to work at a lower temperature, namely about 133 C. for many kinds of steel and iron. I

It has also been found that the solution gradually becomes inactive owing to absorption of carbon dioxide from the air, the caustic soda being thereby converted into inactive sodium carbonate. This should be prevented by covering the tanks with airtight lids when not in use. Also the baths may be regenerated, as far as caustic soda is concerned, by adding lime; this precipitates in- A useful addition to the 3 soluble calcium carbonate and the caustic soda is thereby regenerated. It has been found preferable to replenish the solution with a mixture of caustic soda, sodium silicate, sodium nitrate, lime and manganese dioxide.

Finally it has been found that the rust preventing eifect of the bath may be greatly improved by afterwards dipping the articles for 10 minutes into an acid solution of an oxidizing agent such as a 5% solution of sodium'bichromate.

Articles rust-proofed by the'p'resent process haveatendency to eiiloresce, probably owingto the fact that the caustic soda has not been thoroughly rinsed away. This will also be prevented by the above after treatment. Another way of preventing this is by dipping the treated articles in a dilute solution of acetic or other acid.

The process can be accelerated by heatin the articles before immersing them in the bath preferably by first immersing them in a preheating bath such as a bath of boiling aqueous liquid.

If a bulk of coldmeta-l is immersed in the hot processing solution there is a sudden drop in temperature of the solution. which considerably slows down the reaction and may detrimentally affect it. The preheating 'is preferably done by immersing the ferrous articles in a bath of liquid heated to a suitable temperature. The liquid may, for example, be water, but is preferably a weak acid such as a weakaqueous solution of phosphoric acid. The use of such weak acid in thepreheating bath accelerates the action of the oxidizing agent in the subsequent bath. By using a weak acid as the preheating medium the articles are to some extent pickled and the resulting removal of rust facilitates the subsequent processing.

The following are examples of the compositions of some baths which may beused.

(In place of urea,1tan'nic acid can be used but the percentage should be about one quarter of that of urea, 'theIotherfproportions being correspondingly adjusted!) Water.

In the above examples, all percentages are by weight.

Baths having the compositions shown in Examples 1 to 4 above can be replenished by the addition of mixtures of the same ingredients in the proportions shown, the replenishing mixtures containing, however, in addition 27 parts by weight of lime and 34 parts by weight of man- 'ganese dioxide for every 139 parts by weight of mixture.

In treating ferrous articles they are first dipped in a preheating bath, which may consist of boiling water or boiling weak acid and then left in a bath of the composition shown in any of the above examples at a temperature of to Cuuntil theyhave acquired a jet black color. This generally requires about 20 minutes. They are then dipped in a rinsing bath of boiling If the articles treated are such that small quantities of the treatment solution may be trapped, they may be clipped in a final bath of weak acid (e, g. a 5% solution of acetic acid).

The articles treated must .be free from rust, scale and resinous coatings. Heavily greased articles must be degreased before treatment but lightly greased articles may be treated directly. Acid pickling is required only if mill scale is present.

Steel tanks may be used for the-baths and theirsize will naturally depend uponthe articles treated. Long, shallow tanks are preferable to deep wide tanks.

In use, water may be lost from "the treatment bath. This should be made good, preferably by transferring liquor from the water rinsing tank to which in turn fresh water may be added. Moreover, the other ingredients of the bath are to some extent used up by the treatment of ferrous articles, this being indicated by an increase in the time required to produce a jet black finish or :difficulty in raising the temperature to 127 C. This can .beremediedby adding a replenishing mixture of the com-position indicated after the foregoing examples.

The treated articles should have :an even dead black color. ;'If they have ,little-or-no color or if the finish is not durable :it :isan indication that the temperature of the bath I or its concentration have fallen'too low; in the latter case, replenishment is required. If thearticles have a yellowish-green or reddish layer like-rust or if the treatment liquor becomes thick and foams it is an indication that the temperature of the bath is too high oritsconcentration is-too highyin the latter case dilution is required. Poor coloring may also result if the @articles are not leftlong enoughin the bath.

Thearticles after treatment may be given any desired finish e. g. they may simply be greased or may be coated e.-g. with paint or varnish.

I declaretha-t what I claim is:

1. Processfor treating ferrous-articles capable of rusting to inhibit the development of rust thereon which consists :in forming an oxide film thereon, :integral with the -rnetal by subjecting them ata temperature of between 100 C. and 200 C. toan aqueous alkaline bath containing from 23.5%-to14=2:5% solidcausticsoda, 3.25% to 15% of alkali gsilicate, 2.33% to 9.5% of an oxidizing agent selected from the group-which consists of alkalimetal nitrate, alkali metal nitrite, alkali metal chlorate. :alkali metal permanganate, alkali :metal'bichromate, alkali metal peroxide ;and nitrophen ol, thebalanoe being an amount of water :rather less than the weight of the -,s. 1ids.

2. Process for treating ferrous articles capable of rusting to inhibit the development of rust thereon which consists in forming an oxide film thereof, integral with the metal by subjecting them at a temperature between 100 C. and 200 C. to an aqueous alkaline bath containing from 28.5% to 42.5% solid caustic soda, 3.25% to 15% of alkali silicate, 2.33% to 9.5% of an oxidizing agent selected from the group which consists of alkali metal nitrate, alkali metal nitrite, alkali metal chlorate, alkali metal permanganate, alkali metal bichromate, alkali metal peroxide and nitrophenol, the balance being an amount of water rather less than the weight of the solids, and then dipping the articles into a bath containing a weak acid.

3. Process for treating ferrous articles capable of rusting to inhibit the formation of rust thereon which comprises preheating said articles, then subjecting them at a temperature between 100 C. and 200 C. to an aqueous alkaline bath containing from 28.5% to 42.5% solid caustic soda, 3.25% to 15% of alkali silicate, 2.33% to 9.5% of an oxidizing agent selected from the group which consists of alkali metal nitrate, alkali metal nitrite, alkali metal chlorate, alkali metal pertil 6 manganate, alkali metal bichromate, alkali metal peroxide and nitrophenol, the balance being an amount of water rather less than the weight of solids.

GERSON KURT BERGSTEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Chemical Abstracts, volume 29 (1935), page 6868. 

